1. Field of the Invention
This invention generally relates to a vehicle antenna apparatus, which is mounted on a vehicle and used in an intelligent transport system (ITS). More particularly, this invention relates to a vehicle antenna apparatus, which can prevent a reception level from being reduced owing to antenna directivity.
2. Description of the Related Art
FIG. 10 is a diagram illustrating the entire configuration of a conventional vehicle antenna apparatus, which is described in, for example, Japanese Unexamined Patent Publication No. 64-89601 Official Gazette. As shown in FIG. 10, a vehicle 1 has door mirrors 2 and 3 that contain antennas 4 and 5, respectively. The antennas 4 and 5 are electrically connected to a communication device 6. On the other hand, a road device 8 provided on a road shoulder is electrically connected to a road antenna 7.
In the conventional vehicle antenna apparatus of such a configuration, the two antennas 4 and 5 acting as vehicle-mounted loop antennas are embedded in the two door mirrors 2 and 3 of the vehicle 1, respectively. Output signals of the antennas 4 and 5 are inputted to the communication device 6 through an input circuit (not shown) adapted to add the output signals. On the other hand, the road antenna 7 is installed on a post and mounted high above a road and connected to the road device 8.
The conventional vehicle antenna apparatus of such a configuration has the antennas 4 and 5, which are respectively contained in the two door mirrors 2 and 3, and has the input circuit adapted to add output signals of these antennas. Consequently, this conventional vehicle antenna apparatus achieves stable road-vehicle communication. Moreover, the appearance of the vehicle is not marred. Furthermore, the antennas 4 and 5 are mounted on the outside of a vehicle body covered with iron and glass. Thus, there is little wave loss.
FIG. 11 is a top view of a vehicle, which illustrates the directivity of the conventional vehicle antenna apparatus. Further, FIG. 12 is a side view of the vehicle. In FIGS. 11 and 12, solid lines indicate the directivities of the antennas 4 and 5, namely, the ranges of reception levels thereof in specified planes, respectively. As is apparent from these figures, the directivities or reception levels of the antennas 4 and 5 have maximum values in the travelling direction of the vehicle, respectively, and vary at every predetermined angle in a horizontal plane. Further, the reception levels of these antennas are zero at some angles.
Generally, the antennas 4 and 5 have the aforementioned directivities. Thus, the reception levels reduce in the case that, as the vehicle travels, signals outputted from the road antenna 7 are transmitted from, for instance, directions of arrows drawn with dotted lines in these figure. Consequently, the antennas 4 and 5 cannot receive such signals.
In the conventional vehicle antenna apparatus of such a configuration, the antennas 4 and 5 have the directivities. Consequently, this conventional vehicle antenna apparatus has a problem in that the antenna gains of the antennas 4 and 5 are lowered owing to the directivities thereof and that the reception levels thus decrease.
This invention is accomplished to solve the problem of the conventional antenna apparatus.
Accordingly, an object of the present invention is to provide a vehicle antenna apparatus that can prevent the antenna gain of each antenna from decreasing owing to the directivity thereof and can obtain stable input power.